Solvent extraction using a water-immiscible extractant to remove one ion selectively from others in aqueous solution is a known commercial process. Solvent extraction to remove cobalt from nickel in solution using esters, salts and other organic derivatives of phosphoric, phosphinic and phosphonic acids as selective extractants is known. Generally, alkyl esters of alkyl phosphonic acid and phosphinic acid, which are described for example in U.S. Pat. Nos. 4,196,076, 4,242,314, 4,246,240, 4,348,367, 4,353,883 and 4,382,016 are regarded as being the most selective cobalt extractants. Even with a highly selective extractant, however, more or less nickel will coextract with the cobalt and the extraction circuit will provide for operations to remove such nickel, to regenerate the extractant and to recycle recoverable metal ions.
The extractants themselves are usually dissolved in varying proportions in water-immiscible solvents such as kerosene. The operations involved include extraction in which the raw feed solution containing nickel and cobalt is contacted countercurrently in one or more stages with the extractant to yield a cobalt-loaded extractant and a purified nickel sulfate raffinate which is removed from the circuit. Usually a scrubbing operation is performed on the loaded extractant to remove coextracted nickel and possibly other ions from the cobalt-loaded organic extractant. The scrubbed organic is then stripped with acid solution to yield an aqueous cobalt solution which is removed from the circuit, and to recover the extractant for re-use to extract more cobalt. Aqueous scrub solutions containing cobalt and nickel ions are returned to the extraction step.
The circuit is designed to be operated continuously over long time periods to provide nickel sulfate and cobalt sulfate solutions of sufficient purity that they may be treated for metal recovery by means such as hydrogen reduction to provide metal of commercial purity. In such lengthy operation it is found that alkaline earth metal ions, e.g., calcium and magnesium, present in the raw nickel sulfate solution (which may be, for example, obtained by sulfuric acid leaching of a lateritic nickel ore or oxidation leaching of nickel matte) causes buildup of a hard, adherent and coherent scale on the interior faces of equipment. The scale can build up to the extent of blocking lines, interfering with pumps, interfering with heat transfer, and being in other ways harmful and undesirable. Scale buildup can cause shutdown of plant in order to effect removal. Scale buildup is accordingly an expensive and vexing problem. It is also desirable for economic reasons to minimize recycle of cobalt. It is to the solution of these problems and associated problems that the present invention is directed.
It is known that the extraction process itself operates to transfer selectively across an organic-aqueous interface ions present in the aqueous phase. The organic and aqueous phases are accordingly intermixed to provide a large interfacial area across which the selected ions may pass. After contact of the phases, they are settled in quiescent fashion to achieve separation of the organic and aqueous phases. Equipment designed to accomplish the foregoing is used as is known in the art.